1. Field of the Invention
This invention relates to the field of seismic data acquisition and processing. More particularly, this invention relates to a method of using 2-dimensional extrapolation techniques for predicting lines of seismic data beyond existing lines of seismic data and interpolating lines of seismic data between existing lines of seismic data.
2. Background of the Art
3-D marine seismic surveys entail towing a swath of elongated seismic sensor arrays. The swaths are repeated to increase the amount of coverage in a direction transverse to the direction of the sensor arrays. In order to reduce the cost of acquisition, it is desirable to avoid overlap of successive swaths and, if possible, not leave gaps between the successive swaths. It is an increasingly common practice in marine seismic acquisition to use two or more sources spaced apart in an inline direction to obtain a range of offsets greater than that obtainable without spaced apart sources. Shorter cables have the advantage of increased manoeuverability and less degradation of data coverage due to cable feathering.
U.S. Pat. No. 6,292,755 based on U.S. patent application Ser. No. 09/754,135, a continuation of U.S. patent application Ser. No. 09/337,116 (now abandoned) to Chambers et al, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discloses a method for interpolation and extrapolation of cables of seismic data that may be applied to any form of seismic operation, be it on land or on sea. The teachings of Chambers are applied herein to the problem of maintaining uniformity of coverage in multiple source marine seismic data acquisition.
In 3-D marine operations, a seismic ship tows a swath including a plurality of parallel seismic streamer cables along a desired line of survey, the cables being submerged by a few meters beneath the water surface. The number of cables that make up a swath depends only on the mechanical and operational capabilities of the towing ship. There may be six or more such cables, spaced about 50 to 100 meters apart. The respective cables may be up to 8 to 12 km. long.
Each streamer cable typically includes 120 or more spaced-apart seismic detector groups. Each group consists of one or more individual interconnected detectors, each of which services a single data channel. The group spacing is on the order of 25 to 50 meters longitudinally along the cable. The seismic detectors are transducers that perceive the mechanical activity due to reflected acoustic wavefields and convert that activity to electrical signals having characteristics representative of the intensity, timing and polarity of the acoustic activity as is well known to the art. The detectors are operatively coupled to data-storage and processing devices of any desired type.
An acoustic source such as an array of air guns, is towed in the water by the ship near the leading end of the swath of seismic streamer cables. As the ship proceeds along the line of survey, the source is fired (activated) at selected spatial intervals, commonly equal to the group interval. Assuming the ship travels at a constant velocity such as 4-6 knots, the source may be conveniently fired at selected time intervals such as every 5-13 seconds or more. The wavefield emitted by the source, travels downwardly to be reflected from subsea earth formations whence the wavefield is reflected back to the water surface where the reflected wavefield is received by the detectors and converted to electrical signals. The detected electrical signals are transmitted to any well-known signal recording and processing means for providing a physical model of the subsurface.
FIG. 9 shows an example of seismic data acquired using a two-boat seismic operation is illustrated in which a swath of coverage 613 is obtained using a plurality of cables C1, C2, C3 . . . towed by a first boat 614. FIG. 10a shows the resulting CMP coverage obtained using the acquisition geometry of FIG. 9. A major problem with the CMP coverage is evident: half the lines of CMP bins have a range of offsets that corresponds to the full length of the cable while the other half of the lines have a maximum inline offset that is one-half of the cable length. The crossline positions of the sources could be adjusted so that all the lines of CMP bins would have the full range of offsets; however, such a configuration typically leads to duplicate coverage of many CMP bins, something that is obviously inefficient.
Interpolation of data between existing cables is one possible solution to the problem. The present invention addresses the need for such extrapolation and interpolation of cables of seismic data.
In order to provide a larger range of offsets in marine seismic data acquisition, two spaced apart boats are commonly used. However, with many acquisition geometries, a full range of offsets is not obtainable in all CMP bins. The present invention is a method for extrapolating and interpolating seismic data acquired with such a geometry to obtain a full range of offsets. At each frequency slice of a space gate, a 2-D prediction error filter that can predict the data in forward and backward directions is designed. A prediction filter is obtained from the prediction error filter and applied to a cable at an edge of the space gate to predict a first missing cable. By repeating this process using overlapping inline gates, overlapping inline gates of the extrapolated cables may be obtained. By suitable weighting of the inline gates of the extrapolated cables, a complete cable length is extrapolated. The process may be repeated using the first extrapolated cable in the derivation to give additional extrapolated cables. A similar prediction process is used to interpolate missing cables between existing swaths of 3-D seismic data. Another embodiment of the invention is used for interpolating and extrapolating missing data in a multiple boat acquisition geometry where seismic sources are deployed from at least two spaced apart boats to obtain longer source-receiver offsets